EP-B-0 772 514 discloses self-cleaning structures of articles having an artificial surface structure of elevations and depressions. The distance between the elevations ranges from 5 to 200μ, and the height of the elevations ranges from 50 to 100μ. In addition, at least the elevations are of water-repellent polymers or materials rendered permanently water-repellent. The elevations are not dissolvable by water or water containing detergents.
That solution exhibits a surface having elevations which repel contaminants. A lotus leaf structure is imitated which is known not to be contaminated as a result of self-cleaning, the biological structure of which repels even commercially available adhesives. Despite the remarkable results with respect to self-cleaning effect, the surfaces may be used only to a limited extent, in that either the range of materials to be used in manufacture is greatly restricted or the surface must undergo costly finishing for the purpose of waterproofing. In addition, the process of manufacturing of the disclosed surface is expensive and complicated. Coating processes or shaping processes with high-mesh screens are employed in the manufacture of the disclosed surface which are cost-intensive and difficult to control. Practical experience has shown that “Lotus effect” surfaces produced in this manner often do not yield the desired results as regards self-cleaning.
PCT/WO 93/01047 discloses a surface having a raised thermoplastic film. This surface has a multiplicity of macrocells in the form of elevations extending between these adjacent macrocells. The macrocells have a depth of 0.635μ to 3.81μ. The thermoplastic film has, in addition, at least a plurality of microindentations spaced at intervals ranging from 1.25μ to 6.35μ, that form a randomly distributed sand blast pattern on the film. These microindentations form as an additional structure a second type of elevations having an orientation opposite that of the elevations of the first type, so that the elevations are positioned separately as types on opposite sides of the surface. Such known surfaces, polyolefine foils, for example, such as ones made from polyethylene, with areas of elevations extending between them, are used in particular where special requirements are set for tactile or visual perception, and used for linings for clothing, hygiene or sanitation. Those surfaces possess no antisoiling properties, so that a self-cleaning effect cannot be demonstrated.
EP-A-0 933 388 discloses a structured surface possessing water repellent and/or oil repellent properties, along with low surface energy values. These disclosed surfaces have large water wetting angles. Only with difficulty are they wetted with water to possess a self-cleaning effect. To achieve this effect, a base structure produced by artificial means is provided with two different types of elevations as an additional structure on the surface. Smaller elevations are applied to a superstructure in the form of geometrically larger elevations, which, being immediately adjacent, come in contact with each other. To produce the known elevations and the superstructure as another type of elevations, the latter are simultaneously or in succession mechanically impressed into the surface material, etched in by lithographic processes, or applied by shaping processes or obtained by casting practices. In the case of the mechanical impression process, the effect on the surface is appropriately exerted from the rear side, two types of structural elevations then are formed on its opposite side.
At least some damage to the surface material by the etching agent is to be expected when the structure is etched into this surface material. In the shaping application process, first the elevation structure involved is applied to the surface material by an application roller. This process is expensive and cost-intensive. There is no guarantee that the structure applied will not be separated from the base material again as a function of stress. In addition, the casting, imprinting, etching, and application processes disclosed are not suitable for making large quantities of structured surfaces available in large-scale industrial production. Although this known solution does yield very good results for self-cleaning, its counterpart in nature is in the form of the leaf surface of the nasturtium.